1. FIELD OF THE INVENTION
The present invention relates to a thermally developable light-sensitive material and, more particularly, to a thermally developable light-sensitive material in which heat fog (the disadvantageous blackening of unexposed areas occurring upon thermal development) does not occur.
2. DESCRIPTION OF THE PRIOR ART
It is known to obtain photographic images by subjecting a photographic material containing a light-sensitive element such as silver halide to a so-called dry processing by heating. Of light-sensitive materials capable of forming photographic images through such a dry processing, a thermally developable light-sensitive material utilizing a composition containing an organic acid silver salt (e.g., silver behenate, etc.), a reducing agent and a slight amount of a photocatalyst such as a light-sensitive silver halide as necessary components, e.g., as described in U.S. Pat. Nos. 3,152,904, 3,457,075, 3,707,377, 3,909,271, etc., is at present expected to be the most promising. This light-sensitive material is stable at ordinary temperature but, when heated to usually not less than about 80.degree. C. (preferably not less than 100.degree. C.) after imagewise exposure, the organic silver salt oxidizing agent and the reducing agent undergo an oxidation-reduction reaction under the catalytic action of the exposed photocatalyst such as silver halide present in the vicinity thereof to form silver. Thus, the exposed areas of a light-sensitive layer are rapidly blackened to generate a contrast between the exposed areas and the unexposed areas (background) and form images.
In this light-sensitive system, a photocatalyst such as silver halide remaining in the light-sensitive material after development is not stabilized against light and is allowed to freely discolor. In spite of this lack of stabilization, the same effects as that attained by stabilizing the catalyst can be attained. Because, the photocatalyst such as silver halide is used in such a small amount, with most of the silver salt comprising a stable white or slightly colored organic silver salt which is difficulty blackened with light, that even when a slight amount of the photocatalyst such as silver halide is discolored by light, it appears as a whole white or only lightly colored. Therefore, such a slight discoloration is not visually disadvantageous.
However, one of the defects of the thermally developable light-sensitive materials described in U.S. Pat. Nos. 3,457,075, 3,152,904, etc., is that the unexposed areas tend to be developed as well upon thermal development, i.e., heat fog tends to be generated.
Various techniques are known to prevent generation of heat fog. Such techniques are described in, for example, U.S. Pat. No. 3,589,903, Japanese patent application (OPI) Nos. 10,724/74, 97,613/74, 90,118/74, 22,431/76, U.S. Pat. No. 3,885,968, Japanese patent application (OPI) Nos. 101,019/75, 116,024/75, 123,331/75, 134,421/75, 3,223/76, 42,529/76, 47,419/76, 54,428/76, 57,435/76, 78,227/76, etc. However, these techniques fail to completely solve the problems, and heat fog is not sufficiently depressed.
In addition, conventional thermally developable light-sensitive materials do not have sufficient latitude for thermal development. In particular, thermally developable light-sensitive materials which have been stored for a long period of time after the production thereof tend to have a narrow latitude for thermal development.
Further, when stored for a long period of time after the production thereof, light-sensitive materials in which heat fog is prevented tend to suffer a reduction in sensitivity.